Working with nature to control invasive plants in the Kew Ha-ha

Last month, we worked with CABI (Commonwealth Agricultural Bureau International) and Royal Mid-Surrey Golf Course on a nature-based project to control invasive floating pennywort and protect biodiversity in Kew.

Funded by our Climate Change Steering Group, the project aims to reduce reliance on chemical herbicides and pesticides across the borough. 

The team released Listronotus elongatus (floating pennywort weevils) at the Kew Ha-ha. These small beetles feed on Hydrocotyle ranunculoides (floating pennywort), helping to control the invasive plant and slow its spread.

The Kew Ha-ha is difficult to access, which makes removing invasive plants by hand particularly challenging. Tidal flows can also bring fresh plant fragments into the water. Introducing the weevils offers a cost-effective and sustainable long-term way to manage floating pennywort. Although the weevils will not eradicate the plant completely, they are expected to reduce its growth and spread, limiting its impact on the local ecosystem.

The site will be monitored over the next year to see how well the weevil population becomes established and whether it survives the winter. More weevils may be released if needed. 

Floating pennywort forms dense mats of rounded leaves across the water’s surface. These mats block light, reduce oxygen levels, threaten fish and invertebrates, and crowd out native water plants. Following extensive testing and consultation, the weevil was approved for release in England in 2021 as a biological control for floating pennywort. Adult weevils feed on the leaves and lay their eggs in the plant’s stalks. When the larvae hatch, they tunnel into the stems, reducing the plant’s ability to grow and spread. 

Thank you to our local partners and volunteers whose support made this project possible. Find out how you can volunteer to contribute to the upkeep of local parks and nature conservation sites.